Hitherto, as a frequency tunable oscillator in a microwave band, there is known a frequency tunable oscillator in which a variable capacitance element is provided in a part of an oscillator and the oscillation frequency can be varied by controlling the capacitance through an external electric field (Japanese Patent Application Laid-Open No. 2003-204223).
Referring to FIGS. 11A and 11B, one example of the frequency tunable oscillator disclosed in Japanese Patent Application Laid-Open No. 2003-204223 above will be described. FIG. 11A is a plan view, and FIG. 11B is a cross-sectional view. In the frequency tunable oscillator, a negative resistance element 52 and a CPW (coplanar waveguide) resonator 51 form a feedback circuit via a ground conductor 54, and a variable capacitance diode 53 is mounted parallel thereto. The negative resistance element 52 supplies an electric power to the feedback circuit and resonance is maintained for oscillation. The oscillation frequency depends on the resonant frequency determined by the capacitances of the CPW resonator 51 and the variable capacitance diode 53. Therefore, when a controlling electric field applied to the variable capacitance diode 53 is varied, the capacitance of the variable capacitance diode 53 is varied, thereby enabling the oscillation frequency to be varied. Incidentally, in FIGS. 11A and 11B, reference numeral 531 denotes a capacitor, reference numeral 57 denotes an output line, and reference numeral 58 denotes a dielectric.
However, in the conventional frequency tunable oscillator whose oscillation frequency is selected from the frequency band of from millimeter-wave to terahertz wave, a parasitic reactance component generated when mounting a variable capacitance element may degrade the oscillation characteristics. For example, since the capacitance of the variable capacitance element used in the above mentioned frequency band is small, when a relatively large parasitic capacitance is generated, most of high frequencies will bypass through the parasitic capacitance, so that sufficient capacitance change ratio cannot be obtained. Therefore, the range in which the oscillation frequency can be varied becomes narrow. In addition, there is a case where a parasitic resonance structure including a parasitic inductance generated in a mounted structure may be formed, and in a typical case, the oscillation output will lower.